Acoustic charge transport (ACT) phenomena in III-V semiconductor material has only recently been demonstrated. Such devices have applications as high speed analog signal processors. Delay lines have been fabricated in gallium arsenide (GaAs) devices comprising a surface acoustic wave (SAW) transducer that launch a surface acoustic wave along an upper layer of a GaAs structure that has a transport channel formed therein. An input electrode sources charge to be transported by the propagating electric potential wells and an electrode is present for receiving a signal for modulating that charge. Spaced down the transport channel are one or more nondestructive sensing (NDS) electrodes for sensing the propagating charge. There is also an ohmic output electrode for removing the charge.
Initial acoustic charge transport devices were comprised of a thick epilayer (TE-ACT), with vertical charge confinement accomplished by means of an electro-static DC potential applied to metal field plates on the top and bottom surfaces of the GaAs substrate. The field plate potentials are adjusted to fully deplete the epilayer and produce a potential maximum near the mid-point thereof. Consequently, any charge injected into the channel is confined to the region of maximum DC potential.
Lateral charge confinement (Y direction) has been achieved in several ways. Typically, a mesa is formed to define a charge transport channel. However, for thick epilayer acoustic transport devices, the mesa must be several microns in height, a fact which presents problems in fabrication. Blocking potentials extending down both sides of the delay line have also been used to define the transverse extent of the channel, as has proton bombardment to render the material surrounding the channel semi-insulating.
A heterostructure acoustic charge transport (HACT) device (HACT) has been fabricated using a GaAs/AlGaAs heterostructure that is similar to that of quantum well lasers and heterostructure field effect transistors FET (e.g. HFET, MODFET, HEMT and TEGFET devices). A HACT device is comprised of a sequence of epitaxial layers and vertically confines mobile carriers through the placement of potential steps that result from band structure discontinuities. Besides providing for inherent vertical charge confinement, the HACT devices are thin film devices whose layers have a total thickness of approximately 0.25 microns, excluding a buffer layer. A cap layer is provided with a HACT device both to protect an upper (AlGa)As layer and to permit fabrication of low resistance ohmic contacts and low leakage Schottky metalization.
Prior ACT and HACT devices display only electron transport. For example, a GaAs ACT structure characterized by thick epitaxial material is only capable of electron transport if constructed using n-type thick (5 micron) GaAs layers. In single electron carrier transport device, only half of the acoustic wavelength is used for signal sampling, processing and detection. This fact reduces the sampling frequency and processing bandwidth of the device to one half the frequency of the surface acoustic wave. It would be advantageous to have a acoustic charge transport device which would support both electron and hole transport by the propagating surface acoustic wave, thereby allowing for increased sampling speed and improved device performance. The present invention is drawn towards such a device and also applies to the use of hole transport only.